I'm assuming when they boot up (their copy of) the hard drive, it prompts for a passphrase, and if you type an incorrect one it'll tell that it's incorrect (as opposed to simply outputting garbage). That's the most common case anyway.

If that's the case, and it likely is, then that's pretty concrete proof that there IS something to decrypt.

edit: Actually, the article says that the hard drive was encrypt with Apple's FireVault, so there you go.